III. SIX DIVISIONS OF ARCHITECTURE 75

We will take, first, a short general view of the connection of these members, and then examine them in detail: endeavouring always to keep the simplicity of our first arrangement in view; for protective architecture has indeed no other members than these, unless flooring and paving be considered architecture, which it is only when the flooring is also a roof; the laying of the stones or timbers for footing being paviour’s or carpenter’s work, rather than architect’s; and, at all events, work respecting the well or ill doing of which we shall hardly find much difference of opinion, except in points of æsthetics. We shall therefore concern ourselves only with the construction of walls, roofs, and apertures.

§ 3. (1.) Walls.-A wall is an even and united fence, whether of wood, earth, stone, or metal. When meant for purposes of mere partition or enclosure, it remains a wall proper; but it has generally also to sustain a certain vertical or lateral pressure, for which its strength is at first increased by some general addition to its thickness; but if the pressure becomes very great, it is gathered up into piers to resist vertical pressure, and supported by buttresses to resist lateral pressure.

If its functions of partition or enclosure are continued, together with that of resisting vertical pressure, it remains as a wall veil between the piers into which it has been partly gathered; but if it is required only to resist the vertical or roof pressure, it is gathered up into piers altogether, loses its wall character, and becomes a group or line of piers.

On the other hand, if the lateral pressure be slight, it may retain its character of a wall, being supported against the pressure by buttresses at intervals; but if the lateral pressure be very great, it is supported against such pressure by a continuous buttress, loses its wall character, and becomes a dyke or rampart.

§ 4. We shall have therefore (A) first to get a general idea of a wall, and of right construction of walls; then (B) to see how this wall is gathered into piers, and to get a general idea of piers and the right construction of piers; then (C) to see

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